With respect to calcium carbonate that is one of the constituents forming the composite particles of the invention, there are known ground calcium carbonate and precipitated calcium carbonate. The former ground calcium carbonate is a finely pulverized product of limestone which naturally occurs and has high whiteness, and can be made according to relatively simple processes such as pulverization, classification and the like. The particles have an irregular form inherent to a physically pulverized product and thus a wide size distribution, and can be widely used as a filler or pigment for plastics, rubbers and resins and also for paper-making while making use of their high whiteness and economy.
The latter synthetic calcium carbonate is chemically synthesized calcium carbonate. For the methods of preparing the particles, there are known a carbonation process wherein carbon dioxide is introduced into a calcium hydroxide (slaked lime) slurry for chemical precipitation, a calcium chloride soda process wherein the reaction between calcium chloride and sodium carbonate is used for the precipitation, a soda ash-lime process where the reaction between calcium hydroxide and sodium carbonate is used for the precipitation, and a water treating process wherein the reaction between calcium hydroxide and calcium bicarbonate is used for the precipitation.
Such known processes as mentioned above are now in use for the process of preparing precipitated calcium carbonate. In our country, a starting limestone material of good quality occurs richly, eventually leading to the production using a carbonation process in most cases. In the preparation of precipitated calcium carbonate, it is possible to control a particulate form, a particle size and the like within certain ranges by controlling the preparation conditions including a concentration of calcium in a starting slaked lime slurry, a temperature at which the carbonation reaction is carried out, a rate of carbonation and the like.
It is well known that the particles take various forms particularly including colloidal, cubic, spindle-shaped, columnar and the like forms, and these are, respectively, used for different main applications. More particularly, the colloidal calcium carbonate is in the form of colloidal particles having a particle diameter of 0.04˜0.08 μm and has been used as a filler for plastics, rubbers, paints and the like. The cube-shaped calcium carbonate is in the form of cubic particles with a particle diameter of 0.1˜0.2 μm and is excellent, particularly, as a pigment for paper-making.
The spindle-shaped calcium carbonate is in the form of spindle-shaped particles with a major axis of 0.5˜5.0 μm and a minor axis of 0.1˜1.0 μm and are widely utilized as a filler for paper-making. The columnar calcium carbonate is in the form of columnar particles having a major axis of 1.0˜20 μm and a minor axis of 0.1˜1.0 μm, and mention is made of its application as a pigment, filler for paper-making or the like.
This precipitated calcium carbonate is produced by relatively simple preparation process as set out hereinabove and has prominent properties with respect to physical stability, diversity, the economy in particulate form, whiteness and the like, thus being used in various industrial applications.
The synthetic silica that is the other constituent forming the composite particles of the invention is now described below. For industrial silica materials, mention is made of colloidal silica, silica gel, anhydrous silica, white carbon and the like, which are used in a variety of fields while making use of excellent characteristics such as a high specific surface area, high gas absorptivity, fineness, infiltration or adsorption power into fine interstices, high adhesion, high oil absorption, uniformity of particles, high dispersability and the like.
Of these, colloidal silica consists of amorphous silica, which is obtained by removing impurities from a silicic acid compound to provide a sol of silicic anhydride and controlling the pH and concentration to stabilize the sol and which has spherical, chain-shaped and irregular forms. These have been applied as a processability improver of a resin, a wax, a sizing agent, a quality improver of a latex, a binder, a printability improver for printing paper, a metal surface treating agent, or the like.
A silica gel consists of anhydrous silicic acid obtained by decomposing sodium silicate with an inorganic acid and has been used for applications as a desiccant for foods, medicines, fibers, gases or air, a catalyst or a carrier therefor, a filler for rubbers, or a thickening or precipitation inhibitor for paints or inks. Anhydrous silica is obtained by hydrolyzing silicon tetrachloride and is utilized for a filler or reinforcing agent for paints, inks, resins, rubbers and the like.
Among inorganic materials for industry, calcium carbonate and silica materials are those materials that are most widely employed, and, respectively, have excellent characteristics, also have demerits. For instance, where calcium carbonate is used as a filler for rubber, its surface is so inert as to be poor in affinity for rubber molecules physically and chemically, thus little reinforcing effect on rubber products is obtained.
When used as a pigment or filler for paper-making, particularly for printing paper, the carbonate is lower in ink absorption than synthetic silica, with the possibility that troubles may occur with respect to the ink setting property, strike-through of an ink and opacity of a printed portion. In addition, because of the poor resistance to an acid, it is difficult to use it in combination with an acidic substance such as in an acidic paper-making procedure using aluminum sulfate.
Moreover, where the silica material is employed as a pigment for paper-making, it causes a coating agent to be increased in viscosity, thus making it difficult to formulate in the coating agent at a high concentration. It has been indicated that upon use as a filler for rubber, the viscosity of the resultant rubber composition becomes very high.
Colloidal silica may produce problems with respect to the variations in the temperature, pH, concentration of an electrolyte of a solution, the long-term storage, the stability against organic solvents and the like. In addition, colloidal silica is more expensive than calcium carbonate, thus disenabling one to formulate the colloidal silica in a product in high concentration or use it in large amounts.
Hence, in order to solve or reduce the deficiencies of calcium carbonate, studies have been made from old on the techniques of making a composite product of calcium carbonate and silica and also on applications thereof, under which many proposals have been made.
For instance, Japanese Patent Publication No. Sho 60-72963 proposes a composite improved pigment wherein the surfaces of calcium carbonate particles are activated with an inorganic acid, and silicic acid or a silicate is reacted on the surfaces to form a covering layer of silicic acid or the silicate via CaSiO3 formed by the reaction.
Japanese Patent Publication No. Hei 4-63007 proposes a method of making a specific type of composite powder made of a powder, such as of calcium carbonate and hydrous silicic acid, which is obtained according to a physical technique wherein a mixture of a powder, such as of calcium carbonate and hydrous silicic acid, is ground and which is provided with an ink-strike-through preventing property and is thus suitable as a filler for paper. Japanese Patent Publication No. Hei 11-107189 proposes a method of making composite particles wherein fine particles of calcium carbonate or the like are dispersed in an alkali silicate solution, to which a mineral acid is added under specific conditions to cause the fine particles of calcium carbonate or the like to be uniformly incorporated in the particles of the hydrous silicic acid.
Moreover, applications of the composite product of silica and calcium carbonate include, for example, a carrier for agricultural chemicals (Japanese Laid-open Patent Publication No. Sho 60-222402), a formulating agent for thermal paper (Japanese Laid-open Patent Application No. Sho 61-118287), a pigment for ink jet recording paper (Japanese Patent Publication No. Hei 8-1038), a filler for reinforcing rubber or the like (Japanese Laid-open Patent Application No. Hei 11-29319), and the like. In these publications, there may be exemplified preparation methods or structures different from those of the above-stated composite product.
For instance, the composite material set out in Japanese Laid-open Patent Application No. Sho 60-222402 is such that a metal such as Zn, Mg, Al or the like is caused to co-exist in the course of a carbonation reaction, thereby permitting the metal to co-exist in the resultant composite material, and the composed material described in Japanese Laid-open Patent Application No. Sho 61-118287 is one obtained by co-precipitation of sodium silicate and a water-soluble calcium compound, such as calcium hydroxide, through a carbonation reaction. The composition material of Japanese Patent Publication No. Hei 8-1038 is one obtained by sodium silicate and calcium chloride, followed by carbonation reaction.
These techniques can achieve an effect to some extent in respect of the fact that the defects of calcium carbonate are covered up while making use of the characteristics inherent to silica. However, except for the physical technique disclosed in Japanese Laid-open Patent Application No. Hei 4-63007 or Japanese Laid-open Patent Application No. Hei 11-29319, the composite materials are ones wherein silica is precipitated on the surface of calcium carbonate in a system for coverage with the silica film, calcium carbonate and silica are subjected to co-precipitation, and calcium carbonate is formed, followed by carbonation. There are no cases such that fine particles of silica are added to from outside the system to permit the fine particles of silica to be attached to and fixed on the surfaces of the calcium carbonate.
Problems to Solve
Under the circumstances stated hereinabove, the formation of the composite material by the chemical techniques known up to now differs from the manner of fixing silica particles on the surfaces of calcium carbonate particles as they are. Eventually, it cannot be expected that the excellent characteristics of fine particles of silica are imparted thereto, and thus, such characteristics can be shown only within a limited range.
We have made intensive studies on the formation of a composite material of synthetic silica and calcium carbonate for the purpose of realizing the high functionality of calcium carbonate, with the result that the composite material of the invention can be developed.
Accordingly, the invention has for the problem to be solved the provision of novel silica-calcium carbonate composite particles without a sacrifice of excellent characteristics of silica, such as a high specific surface area, high gas adsorbability, fineness, high infiltration into fine interstices and high adsorption, high adhesion, uniformity of particles, high dispersability and the like, and excellent characteristics of calcium carbonate, a paper sheet having the particles internally filled therein or coated therewith, and a method for making the particles.
More particularly, the invention has for its object the provision of novel silica-calcium carbonate composite particles having excellent characteristics inherent to both synthetic silica and calcium carbonate, a paper sheet having the particles internally filled therein and/or coated therewith, and a method for making the particles.